Of particular interest for the invention are, in general, all types of sensors in which an excessively high voltage between the connections of the sensor bring about the danger of impairing the sensor function and/or the danger of irreversible damage to the sensor. This is, for example, the case for many gas sensors in which operational voltages are applied to a ceramic material via the sensor connections or currents are allowed to flow through this ceramic material. Such sensors can be impaired or damaged if too high voltages are applied to the ceramic material. Therefore, measures have usually been taken for well-known circuit arrangements to operate such sensors to avoid the occurrence of excessively high voltages at critical points of the sensor.
A circuit arrangement with the features of the preamble of claim 1 is well-known. In the case of this well-known circuit arrangement for operating a linear exhaust gas sensor for an internal-combustion engine, a control circuit is provided to supply the exhaust gas sensor via a plurality of connecting lines with an electrical pump current and to detect this pump current as well as a sensor voltage as electrical output signals of the exhaust gas sensor. In order to protect the exhaust gas sensor from destruction by applying a disproportionately high voltage at a pump cell loaded with a pump current as well as at the sensor, the control circuit detects an electrical potential at the connecting lines by means of comparators and, in the case of a disproportionately high voltage, activates the switching transistors to interrupt the connecting lines. Such impermissibly high voltages can, for example, occur in the case of a short-circuit with a voltage-carrying line situated in the vicinity and without this controlled interruption of the connecting lines could damage the pump cell or sensor of the exhaust gas probe.
However, damage to the sensor is not totally excluded even when this known circuit arrangement is used. On the one hand, this known suppressor circuit is only effective in operation because after the supply voltage has been switched off, abnormal potentials are not detected and there might be no interruption of the connecting lines. In the case of a motor vehicle that uses the known circuit arrangement to operate an exhaust gas sensor, the above-mentioned interruption function is no longer provided after, for example, the engine of the motor vehicle has been switched off. Even when the circuit arrangement is in operation, optimum protection is not available because the switching transistors used to interrupt the connecting lines have a finite electrical resistance even in the opened state so that, depending on the type of overvoltage applied, there may still be an impermissibly high voltage at the sensor. This problem is all the more serious, the greater the internal resistance of the sensor between the relevant sensor connections, since an abnormal potential occurring in the area of the connecting lines in the case of a high internal sensor resistance drops off at this very internal resistor. As is well-known, a ceramic material (e.g. zirconium ceramic) used to construct a linear exhaust gas probe has a resistance that strongly depends on the temperature. In the cold state of the sensor, e.g. immediately after starting the internal-combustion engine, the internal resistances of the sensor developed by the ceramic materials are extremely high so that even small leakage currents flowing over the switching transistors can give rise to particularly high voltage drops at the sensor.